There isn’t much in Kamchatka, a remote peninsula in northeastern Russia just across the Bering Sea from Alaska, besides an impressive population of brown bears and the most explosive volcano in the world. Kamchatka’s Shiveluch volcano has had more than 40 violent eruptions over the last 10,000 years. The last gigantic blast occurred in 1964, creating a new crater and covering an area of nearly 100 square kilometers with pyroclastic flows. In fact, Shiveluch is currently erupting, as it has been for over 20 years. So why would anyone risk venturing too close?
Michael Krawczynski, assistant professor of Earth and planetary sciences, has seen the awesome power of Shiveluch firsthand. He and his team brave the harsh conditions on Kamchatka because understanding what makes Shiveluch tick could help scientists understand the global water cycle and gain insights into the plumbing systems of other volcanoes. They published the results of their investigation, “Evidence for superhydrous primitive arc magmas from mafic enclaves at Shiveluch volcano, Kamchatka,” Nov. 18 in the journal Contributions to Mineralogy and Petrology.
Though scientists aren’t certain exactly how much water is moving in and out of Earth’s interior, they know it must be balanced. “Subduction is bringing water back into the mantle. If you had nothing coming back out, eventually your oceans would just get sucked down into the mantle,” Krawczynski explained. “So, because that doesn’t happen, we know it’s a balanced cycle. We’re studying these volcanoes to find out how much water is coming back out to understand how much water is going in.”
Lead author Andrea Goltz, a graduate student working with Krawczynski, described Shiveluch as an extreme case in terms of its water content. Volcanoes like Shiveluch form at convergent margins where two tectonic plates meet and one slides under the other in the process of subduction. There is volcanism at convergent margins because water released from the subducting plate as it is pulled down into the Earth’s mantle decreases the melting point of the mantle, producing magmas. Though water is central to the formation of magmas at subduction settings, the amount of water dissolved in magmas at subduction settings is variable.
“This volcano, Shiveluch, is known to be especially hydrous. It’s an extreme case on the global subduction scene,” Goltz said. As magma travels up through the crust, it changes its composition, including its basic chemistry and water content. “Lots of researchers have looked at more evolved magma compositions at shallower depths, but less work has been done on the original starting water composition of less evolved (or more primitive) magmas. What we’ve done in this paper is to quantify this volcanic extreme.”
Quantifying the water content of primitive magmas before they have changed too much in composition tells scientists about processes involved in the formation of Shiveluch and other volcanoes like it. Knowing how much water is coming out of Earth’s interior through volcanoes is an important part of understanding the global flow of water. But, it’s a rare thing to have pristine, unaltered magmas erupting at the surface where researchers can sample them.
The process of cooling, crystallization, and eruption usually destroys the pristine, primitive nature of magmas and makes it difficult to estimate their water contents. Instead of targeting the voluminous products of volcanic eruptions, Goltz and her collaborators take a different approach. “In this study, we looked at small nodules of primitive magma that were erupted and preserved amid more evolved and voluminous material. The minerals in these nodules retain the signatures of what was happening early in the magma’s evolution, deep in Earth’s crust.”
Getting a glimpse of the deep, inner workings of Shiveluch is possible in part because it is so active. Though scientists can’t venture too close to the vent of the volcano itself during field work, the high output from Shiveluch has provided numerous samples from different eruptive events over time, which can be gathered from a relatively safe distance away from the active crater. Krawczynski pointed to one spot, for example, where researchers could sample many different lava flows or ash fall deposits, gaining access to the volcanic record without having to climb into the crater.
Earlier work from Krawczynski’s lab established limits on how much water content might be captured by crystal melt inclusions in erupted magmas. This new study complements that work by going as close as possible to the original material and determining how much more water it might have contained, beyond what can be preserved in a melt inclusion. Of particular interest is a mineral called amphibole, which acts as a proxy or fingerprint for high water content at known temperature and pressure. The unique chemistry of the mineral tells researchers how much water is present deep underneath Shiveluch.
Spoiler alert: it’s a lot.
“Amphibole is special in that it likes to crystallize from primitive melts at relatively low temperatures and relatively high water contents,” Goltz explained. “We know it’s crystallizing early in primitive magmas because we’re finding it inside another mineral called olivine. By themselves, olivine and amphibole are common, but finding them together is really rare. That co-existence in the same magma limits the possible temperature of primitive magmas at Shiveluch and requires high water content.”
The conditions inside Shiveluch include roughly 10-14% water by weight (wt%). Most volcanoes have less than 1% water. For subduction zone volcanoes, the average is usually 4%, rarely exceeding 8 wt%, which is considered superhydrous.
“When you convert the chemistry of these two minerals, amphibole and olivine, into temperatures and water contents as we do in this paper, the results are remarkable both in terms of how much water and how low a temperature we’re recording,” Krawczynski said. “The only way to get primitive, pristine materials at low temperatures is to add lots and lots of water. Adding water to rock has the same effect as adding salt to ice; you’re lowering the melting point. In this case, there is so much water that the temperature is reduced to a point where amphiboles can crystallize.”
Goltz’s results prove what Krawczynski’s earlier study suggested was possible – magmas with very high water contents do exist. How common such superhydrous magmas are remains an open question. Though Shiveluch is known to be a special case in terms of its activity, it is possible there are superhydrous magmas in volcanoes all over the world. Geochemists might simply have no access to telltale chemical signatures if they aren’t preserved through eruption.
“That’s such an important question for the global water cycle, but it’s not even a question that you could ask until we did this work and showed that these things do exist,” Krawczynski said.
“Looking at something this extreme can inform how we look at other volcanoes,” Goltz added. “It expands the imagination and the limits of human exploration.”
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